Php 无法为类内的静态变量赋值

我试图为一个类实现单例模式。我有一个静态变量$classInstance，分配类对象时出现错误“Parse error:syntax error，unexpected'=”

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错误是因为使用self关键字访问类的静态属性时缺少“$”

  /**
   * Create the client connection.
   */
  public static function createClient() {
    if (self::$classInstance === null) {
      self::$classInstance = new self(); // Note the $ sign.
    }
    return self::$classInstance;
  }

  /**
   * Create the client connection.
   */
  public static function createClient() {
    if (self::$classInstance === null) {
      self::$classInstance = new self(); // Note the $ sign.
    }
    return self::$classInstance;
  }